1. Field of the Invention
The present invention relates to methods and apparatus for controlling the penetration rate of a drill bit and more particularly to such methods and apparatus which use a fluidic ram to vary the weight applied to the drill bit.
2. Setting of the Invention
In the drilling of oil and gas wells, a common equipment configuration includes a drill bit mounted on the lower end of a downhole hydraulic motor (such as a turbine or positive displacement motor) which is suspended from the lower end of a string of drill pipe. Drilling fluid is pumped through the drill pipe to the motor. The motor is powered by fluid flowing through it and thereby rotates the drill bit. The drilling fluid continues its downward flow through openings in the lower end of the drill bit. The drilling fluid serves to power the motor as well as to cool the drill bit and to flush cuttings from the bottom of the wellbore upwardly to the surface in the annulus between the drill string and the bore.
When using a downhole motor of the turbine type in the above-described configuration, it is desirable to operate the downhole motor at its maximum power. Such operation occurs when the downhole motor output shaft or rotor is spinning at approximately one-half of its runaway or no-load speed. When so operating, the downhole motor is applying a constant torque to the drill bit and this is true regardless of variations in rock properties and bit wear.
There is an approximately direct relationship between the torque delivered to the drill bit and the amount of weight applied to the bit. The drill bit weight required to maintain the desired torque level varies with changes in rock properties and drill bit dullness. Thus, the downhole motor may be operated at its maximum power by maintaining the torque applied to the drill bit at the constant level at which maximum power is delivered.
Several problems are encountered when drilling with a downhole motor as described above. It is difficult to know whether the desired torque level is being applied to the drill bit because there is little indication at the surface of the torque being applied to the drill bit. Also, it is difficult to accurately measure the weight applied to the drill bit at the surface due to wellbore friction acting on the drill string. Although there exist commercially available devices for measuring weight applied to the drill bit and drill bit torque at the bit, when such are used to transmit information to the surface to vary the force used to suspend the drill string (which in turn varies the weight applied to the bit), the response time is insufficient to avert drill bit failure when the property of rock through which the bit is drilling suddenly changes.
In the past, downhole hydraulic rams have been incorporated into the drill string above the drill bit to apply a downward force to the bit. Such past hydraulic rams are pressurized by a pump powered by the downhole motor or by the pressure of the drilling fluid in the drill string and are thus not acting directly in response to torque applied to the drill bit. Moreover, such past hydraulic rams are pressurized to urge the bit downwardly relative to the drill string and thus prevent an accurate measurement of the weight applied to the bit at the surface. In addition, it is difficult to determine at the surface of the well when to advance drill pipe because the position of the piston in the ram relative to the drill string cannot be accurately determined.
There exists a need for a method and apparatus for controlling the penetration rate of a drill bit in which the rate is controlled to obtain a constant drill bit torque. Moreover, there exists a need for such a method and apparatus in which the weight applied to the bit, and thus the torque, is varied by a downhole fluidic ram which acts in response to drill bit torque. Further, there exists a need for such a method and apparatus which may be powered downhole without addition of a pump. Finally, there exists a need for such a method and apparatus which provides an indication of when to advance the drill pipe and which permits accurate measurement of weight applied to the drill bit.